Blends with intrinsically conductive polymers, especially with dispersible intrinsically conductive polymers in powder form that are significant for technical applications, are described in U.S. Pat. No. 5,217,649 and PCT/EP88/00798. The definitions and concepts described therein are also applicable to the present application and are, therefore, incorporated by reference herein.
Such blends show conductivities in the range of 10.sup.-9 up to about 2.5 S/cm. The upper limit being the conductivity of the virgin conductive polymer in a dispersible form, which iS typically in the range of 1-10 S/cm. We define herein a dispersible intrinsically conductive polymer (ICP) as capable of being dispersed by conventional means in a liquid matrix or a polymer matrix such that at least 50 percent of the ICP by weight is present at a particle size of less than 500 mm.
For several applications such as electromagnetic interference (EMI) shielding and the like, the conductivity of known intrinsically conductive polymers falls short of commercial interest. For example, it has been shown by Shacklette et al., Journal of Vinyl Technology 14(2), 118, 1992, that in order to achieve 40 dB shielding, which represents a minimum requirement for many commercial applications, a minimum thickness of 3 mm with such conductivities is required. There are also requirements on the mechanical properties of such blends which would prevent the use of more than about 25 percent by volume of a conductive polymer in such a blend. The upper conductivity limit of 1-5 S/cm for blends with acceptable mechanical properties, which has hitherto been impossible to exceed, limits technical applications for such blends.
There is, therefore, a need--not only for applications in the field of EMI shielding--to increase the conductivity of polymer blends with intrinsically conductive polymers. In particular, there is a need to increase the conductivity of blends with polyaniline (in thermoplastic or non-thermoplastic polymers or in paints or other applications).
The term "intrinsically conductive polymer" (ICP) refers to organic polymers containing polyconjugated bond systems such as double and triple bonds and aromatic rings which have been doped with electron donor or electron acceptor dopants to form a charge transfer complex having an electrical conductivity of at least about 10.sup.-6 S/cm by the four-in-line probe method. Examples of such polymers are polyaniline, polypyrrole, polyacetylene, polythiophene, polyphenylene and the like.
There recently has been increased interest in processing of intrinsically conductive polymers into useful conductive materials. Polyaniline in particular has received considerable attention due to its ease of manufacture, environmental stability and moderate conductivity. In its doped form it has a conductivity in the 1-5 S/cm range. See for example, U.S. Pat. No. 5,160,457; 4,069,820; 4,915,164; 4,929,388; 4,983,322; PCT applications WO 89/02155, 90/10297 and Synthetic Metals, volumes 1-57.
In recent years scientists have made considerable efforts to achieve higher conductivities. The following processes have so far been used on a laboratory scale with pure ICP's:
1. Polymerization of polyacetylene in viscous non-polar media, followed by stretching and subsequent doping with iodine (Naarmann and Theophilo, Synthetic Metals 22 1 (1987)). Conductivities of several times 10.sup.4 S/cm have been achieved. The process has the disadvantage that it is difficult to perform, difficult to reproduce and results in a conductive polymer that is not air and oxidation resistant and not processable. Owing to these problems, polyacetylene has remained a laboratory curiosity. PA1 2. Polyprrole can be polymerized under specific electrochemical conditions to films that have a conductivity of several times 10.sup.2 S/cm. This process has the disadvantage that only self-supporting films can be produced which are not processable or dispersible, and are also not sufficiently stable at medium high temperatures. PA1 3. Recently, fairly high conductivities have been reported in polyaniline, see for example Y. Cao et al. (Synth. Met. 48, 91 (1992), Appl. Phys. Lett. 60, 2711 (1992), Y. Cao et al., Synth. Met. 55-57 (1993) 3514-3519. This process involves synthesizing polyaniline protonated ("doped") with hydrochloric acid, neutralizing it to obtain emeraldine and then protonating it again with another acid, in this case preferably camphor sulfonic acid in the presence of m-cresol. The resulting nondispersible self-supporting films possessed a conductivity of about 1.5.times.10.sup.2 S/cm. In addition to their non-dispersibility and the highly complex process by which they are made, a further disadvantage must be seen in the fact that some of the m-cresol remains in the conductive film and potential toxicological problems arise both during the process and during later use. The process is believed to enhance the conductivity via increased crystallinity and solubility, camphor sulfuric acid/m-cresol induced solubility. PA1 4. A. Monkman et al. Solid State Commun. 78, 29 (1991) reported a conductivity of 60 to 70 S/cm for films cast from N-methylpyrrolidone (NMP), which were doped with HCl, and 200 to 350 S/cm when neutral polyaniline films (films of emeraldine base) were stretched and subsequently doped. These films cannot be subsequently processed into any other useful forms and are not dispersible under the definition given herein. PA1 5. Recently, B. Wessling et al. (DE Pat. Application P 43 17 010 2) have shown that a significant increase in conductivity in intrinsically conductive polymers, preferably polyaniline, in raw powder form can be achieved by an additional dispersion process in the pure state, leading to enhanced conductivity values of greater than about 2.5.times.10.sup.1 S/cm. PA1 5. Alternatively, chain alignment of polyaniline has been achieved via solution spinning from concentrated sulfuric acid to produce fibers of polyaniline with enhanced crystallinity and high conductivity in a range from 20 to 60 S/cm (A. Andreatta, Synth. Met. 26, 383 (1988)). PA1 7. It is also known that the conductivity of polyaniline is enhanced through hydration by water. Such a hydrated polyaniline is difficult to process by conventional thermoplastic means since the water has detrimental effect on the thermoplastic polymer as well as the conductivity of the blend or it causes the polyaniline to become insoluble or undispersible in conventional organic solvents. Since the water would be the last from the polyaniline during exposure to elevated temperature (greater than about 30.degree. C.) or during exposure to less humidity conditions (less than 50 percent RH), the conductivity will have less thermal stability and less humidity independence than would a polyaniline complexed with the less volatile polar materials of the present invention.
In summary, the prior art discloses conductivity enhancing processes that are complicated multistage processes and/or those requiring subsequent doping. Furthermore, other fundamental disadvantages still exist in that the resulting products are not further processable or dispersible. For instance, oriented fibers or polyaniline or other conductive polymers must be subsequently used in fiber form to preserve their enhanced conductivity which is produced by chain alignment.
Therefore, there remains a need to create intrinsically conductive polymer blends, with conventional insulating polymer which are processable by conventional techniques such as injection molding, extrusion, calendaring, or the like, and which possess a conductivity of greater than 2.5 preferably greater than 25 and most preferably from 100 to 2.5.times.10.sup.5 S/cm, without complicated multi-step processes or predispersion steps.